Conventionally, for this kind of water treatment system, there has been proposed one that converts raw water such as seawater to mist by heating in a chamber, and condenses steam by causing a vapor-liquid contact between air-steam mixture gas, generated while converting the raw water to mist, and spray water, thereby desalinating or purifying the raw water (for example, refer to Patent Literature 1). This water treatment system efficiently heats raw water by causing heat exchange between desalinated or purified condensed water and raw water such as seawater and then heating the raw water with a heater utilizing solar energy.